I don't fully know, but I suspect it's just that due to the minor faults and tlb misses there is terrible contention with the spinlock, regardless of the PREEMP

I don't fully know, but I suspect it's just that due to the minor faults and tlb misses there is terrible contention with the spinlock, regardless of the PREEMPT_LAZY when using 4k pages (that easily reproducible). Which is then made worse by preempting more with the lock held.